The present invention relates to a belt for a continuously variable transmission, which comprises a metal ring assembly comprising a plurality of endless metal rings laminated one on another, and a large number of metal elements each having a ring slot into which the metal ring assembly is fitted, the belt being wound around a drive pulley and a driven pulley to transmit a driving force between both of the pulleys.
There is an invention relating to such a belt for a continuously variable transmission, which is conventionally known from Japanese Patent Application Laid-open No.7-12177, and in which in order to inhibit the rolling of metal elements supported on a pair of metal ring assemblies to smoothly engage the metal elements in pulleys, a vertical (in a direction of thickness of the belt) clearance in each of the metal ring assemblies within a ring slot in a metal element (a difference between a groove width of the ring slot and a thickness of the metal ring assembly) is defined.
FIG. 11 shows a cross section of the belt for the continuously variable transmission described in the above-described Japanese Patent Application Laid-open No.7-12177. In this belt, metal ring assemblies 03, 03 are engaged in a pair of left and right ring slots 02, 02 in a metal element 01. When the metal element 01 is not rolled, lower surfaces of the metal ring assemblies 03, 03 are in contact with saddle surfaces 04, 04, and a clearance B is established between upper surfaces of the metal ring assemblies 03, 03 and lower ear surfaces 05, 05. When the metal element 01 has been rolled in a direction of an arrow a in FIG. 11, the lower surface of one of the metal ring assemblies 03 is brought into contact with the saddle surface 04 at a point b, and an upper surface of the other metal ring assembly 03 is brought into contact with the lower ear surface 05 at a point c. By setting the clearance B so as to establish an expression, tanxe2x88x92B/A less than 1xc2x0 wherein A represents a distance between the points b and c, it can be prevented that when the metal element 01 has been rolled at least at 1xc2x0, the pair of metal ring assemblies 03, 03 are brought into contact with the saddle surface 04 and the lower ear surface 05 at the points b and c, whereby the metal element 01 is further rolled by a reaction force resulting from such contact.
The belt described in the above-described Japanese Patent Application Laid-open No.7-12177 is designed, so that the rolling of the metal element is inhibited. However, it is known that the clearance between the upper surface of the metal ring assembly and the lower ear surface also exerts a large influence to the durability of the metal ring assembly, when the metal element has been pitched. More specifically, when the metal element has been pitched, an edge of the lower ear surface is brought into contact with the upper surface of the metal ring assembly to generate a stress at such contact portion, thereby reducing the durability of the metal ring assembly. The pitching of the metal element occurs in the vicinity of an exit of a driven pulley by the following reason:
When the metal element 32 supported with the ring slots 35, 35 fitted over the metal ring assemblies 31, 31 has been inclined forwards in a direction of movement, as shown in FIG. 3, the following problem is encountered: Front ends of the lower ear surfaces 45, 45 forming upper edges of the ring slots 35, 35 in the direction of movement are brought into contact with the upper surfaces of the metal ring assemblies 31, 31 (see a mark ◯ in FIG. 3) and hence, a large hertz stress is generated at such contact portion to exert an adverse influence to the durability of the metal ring assemblies 31, 31. The forward inclination of the metal element 32 in the direction of movement is generated by a tangent frictional force F received by the metal element 32 at its surface of contact with the pulley and by an urging force E between the metal elements 32. Such a tendency is particularly remarkable at an exit area of the driven pulley (see FIG. 10). The reason will be described below.
In FIG. 3, a moment generated in the direction of an arrow M by the tangent frictional force F acts to bring down the metal element 32 forwards in the direction of movement about the swinging center C. On the other hand, a radial (vertical) frictional force xcexcE generated by the urging force E between the metal elements 32 generates a moment in the direction opposite from the arrow M in the metal element 32 and acts to bring down the metal element 32 backwards in the direction of movement about the swinging center C.
It is known that the tangent frictional force F received by the metal element 32 from the drive pulley 6 or the driven pulley 11 is large in the exit area of the drive pulley 6 or the driven pulley 11, as shown in FIG. 9A, and the value of such tangent frictional force F reaches four times a value provided when it is supposed that the tangent frictional force F has been averagely distributed over the entire winding region of the pulley 6 or 11, by reason that the pulley 6 or 11 is deformed, resulting in an axial thrust concentrated thereon, or by another reason. In addition, as shown in FIG. 9B, the urging force E between the metal elements 32 has a large value in the exit area of the drive pulley 6, but assumes zero in the exit area of the driven pulley 11. Therefore, the metal element 32 is most liable to be inclined forwards in the direction of movement in a position where the tangent frictional force F acting to incline the metal element 32 forwards in the direction of movement assumes a maximum value and the urging force E acting to inhibit the forward inclination of the metal element 32 in the direction of movement assumes zero, i.e., in the exit area of the driven pulley 11.
The present invention has been accomplished with the above circumstances in view, and it is an object of the present invention to ensure a power transmitting efficiency, while preventing a reduction in durability of the metal ring assembly due to the pitching of the metal element.
To achieve the above object, according to the present invention, there is provided a belt for a continuously variable transmission, which comprises a metal ring assembly comprising a plurality of endless metal rings laminated one on another, and a large number of metal elements each having a ring slot into which the metal ring assembly is fitted, the belt being wound around a drive pulley and a driven pulley to transmit a driving force between both of the pulleys, each of the metal elements having a projection and a recess provided on front and rear surfaces thereof for engaging with preceding and succeeding ones of the metal elements, respectively, characterized in that when the size of a first vertical clearance between the projection and the recess of the metal element is represented by CLNH; the size of a second vertical clearance in the metal ring assembly within the ring slot is represented by CLy; a thickness of the metal ring assembly is represented by TL; and a vertical opening of the ring slot is represented by D, the following expression is established:
(CLNH/TL)+1 less than D/TL less than (0.86 mm/TL)+1 
With the above arrangement, the second vertical clearance CLy in the metal ring assembly within the ring slot is sized to be larger than the first vertical clearance CLNH between the projection and the recess of the metal element. Therefore, when the metal element has been pitched, the first clearance CLNH is first distinguished, whereby a further increase in pitch angle is inhibited, and the second clearance CLy is prevented from being distinguished. As a result, the upper surface of the metal ring assembly is prevented from being brought into contact with an upper edge of the ring slot to avoid a reduction in durability of the metal ring assembly.
When D/TL exceeds (0.86 mm/TL)+1, the power transmitting efficiency is reduced and hence, a reduction in power transmitting efficiency can be prevented by setting D/TL at a smaller value than (0.86 mm/TL)+1.
There is provided a belt for a continuously variable transmission, in addition to the above arrangement, wherein an expression, 1 less than D/TL less than 11/8 is established.
With the above arrangement, a reduction in power transmitting efficiency can be prevented, while avoiding a reduction in durability of the metal ring assembly by setting the ratio D/TL, so that the expression, 1 less than D/TL less than 11/8 is established in a common metal belt having a width of about 24 mm.